The stuff produced by your immune system in its attempt to get rid of (usually) a bacterial infection by the process of suppuration. Pus is mostly made up of phagocytes and bacterial cells - it looks white or yellowish and is icky.

Drainage of a collection of pus is usually a good idea.

Pus is also the word from which the word "pussy" has another meaning that has nothing to do with an English slang for a part of the female genitalia.

Pus (?), n. [L., akin to Gr. , , and to E. foul: cf. F. pus. See Foul, a.] Med.

The yellowish white opaque creamy matter produced by the process of suppuration. It consists of innumerable white nucleated cells floating in a clear liquid.

© Webster 1913.